This invention relates to an integrated circuit and to a method of making an integrated circuit.
Diodes are well known the art for their rectifying properties, whereby current is allowed to pass only in a single direction through the device. With reference to FIGS. 1A and 1B, the properties of the ideal diode and a typical real-world diode are briefly compared.
As shown in FIG. 1A, for V>0 (forward bias), the current in the ideal electrode is essentially an exponential function of V, while for V<0 (reverse bias), no current is allowed to flow through the device.
As is well known, the behaviour of a typical real-world diode deviates from the ideal shown in FIG. 1A in three main ways. These deviations are illustrated in FIG. 1B.
In particular, FIG. 1B shows that below a characteristic forward bias switch-on voltage Vf, the current flowing through the diode for V>0 remains substantially at zero. The current in the device rises exponentially above Vf. An alternative way of viewing the behaviour of the ideal diode shown in FIG. 1A is to treat Vf as essentially equal to zero.
Also, as shown in FIG. 1B, for V<0, a small reverse current (reverse bias leakage current IRB) flows through the diode, such that the diode does not act as a perfect rectifier (current may pass through the diode in either direction, but is limited to IRB in the reverse direction).
Further, as shown in FIG. 1B, and as is also well known, typical real-world diodes exhibit large reverse breakdown currents below a certain reverse bias voltage V<VBR.
The values of Vf, IRB, and VBR depend on the design of the diode, and can be tailored according to design requirements by altering parameters such as the doping levels and work functions of the materials used within the device.
For certain applications, such as in rectifiers for low amplitude AC signals and/or in charge pump circuits, it is desirable to use diodes that have a low value of Vf.
For example, to rectify an AC signal with a small amplitude, a small Vf is necessary in order to have any output signal (Vf should be smaller than the AC signal amplitude).
A more specific example is the charge pump circuit, which can be used for rectification and amplification purposes in an RFID tag. Such a circuit receives a small AC signal with amplitude Vin and rectifies/amplifies it to a value of:Vout=2·n·(Vin−Vf)  (1)where n is the number of charge pump circuits (stages) in series.
From equation (1) it can be seen that in order to have an output voltage, Vf should be lower than Vin. Taking into account that Vin depends on the distance between RF signal emitter and receiver (the longer the distance between them, the smaller the Vin), having a low Vf implies that the RFID can operate at even greater distances. Furthermore, even at a fixed distance between RFID tag and RF signal emitter, a low Vf implies higher power efficiency (how much of the input signal will pass through the diode).
For applications such as those mentioned above, Schottky diodes, which typically do have low values of Vf, are therefore particularly suitable. As is well known, Schottky diodes typically include a metal-semiconductor junction, which forms the basis for the rectifying action of the diode.
To reduce costs and allow easy integration with other devices, it is desirable to be able to fabricate Schottky diodes using existing CMOS processing steps.
However, it is difficult to integrate the provision of a metal-semiconductor contact into existing CMOS process flows. For example, with some exceptions (in highly advanced, more expensive process flows), metal is excluded from use in Front End Of Line (FEOL) wafer processing.
Accordingly, the provision of a Schottky diode into integrated circuit chips or dies manufactured by CMOS processing (e.g. for incorporation into the die of an RFID tag) is difficult to achieve.